Effect of green manures and organic amendments on Verticillium wilt of potato in Manitoba

Verticillium dahliae Kleb. is a soil-borne fungal pathogen of several crops and is broadly distributed worldwide. It is a very persistent soil-borne fungus in potato (Solanum Tuberosum L.) and responsible for the Verticillium wilt of potato. In Manitoba, potato fields planted with cv Russet Burbank: have been found to be infested with highly pathogenic V. dahliae isolates, which can produce disease severity of up to 90% and reduce yield. Potato producers have then an increased interest on use of green manures and organic amendments to control Verticillium wilt. The objectives of this research were to evaluate selected green manure and organic amendments for their ability to reduce propagule density of V. dahliae in soil, incidence and severity of Verticillium wilt, and to enhance potato yield in Manitoba. In addition, a second study was conducted for the purpose of studying the potential of mustard green manure and seed meal at lower application rates than those previously recommended to inhibit the germination of microsclerotia. Our results for the first study showed that compost and seed-meal treatment reduced disease incidence to 30 and 40 % (P <0.001), but only seed meal reduced propagule density. Potato marketable yield increased only with application of compost. Overall, 1- or 2-years green manures were ineffective reducing propagule density or improving potato yield. Application of Vapam was partially effective reducing the propagule density only at the beginning of the potato season, but it did not reduce disease incidence compared to control. Our results for the second study showed that 2-propenyl isothiocyanate (2-propenyl-ITC) was the only ITC present in amended soil with higher concentration in seed meal amendment. The germination of microsclerotia was mainly inhibited by 0.5 and 0.25% mustard seed meal treatments at four-day measurement. However, the effect of concentration of 2-propenyl-ITC at 22 nM g-1 seems to be fungistatic, as germination of microsclerotia rebounded once the concentration of ITC dissipated. The results of this study suggest that the inhibition of germination of microsclerotia by oriental mustard plant tissue or seed meal is affected by soil with pH higher than 6.5, where the concentration of 2-propenyl-ITC is lower and dissipates rapidly. Findings from the current research suggest that one or two year of green manure does not appear to be an effective management tool for Verticillium wilt of potato in Manitoba. Composted beef cattle manure and oriental mustard seed meal amendments have promise as an alternative strategy for the control of V. dahliae. However, in terms of effective field implementation, higher concentration and residence time of 2-propenyl-ITC will be needed to have a fungitoxic effect on V. dahliae microsclerotia with mustard seed meal. Finally, only composted beef cattle manure reduced disease, increased potato yield and improved nutrient availability (P) in soil.